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Arxiv: Unveiling the Cosmic Gems Arc at z\sim10.2 with JWST Published: 4/16/2024 8:54:49 PM Updated: 4/16/2024 8:54:49 PM

Paper abstract: We present recent JWST NIRCam imaging observations of SPT0615-JD (also knownas the Cosmic Gems Arc), lensed by the galaxy cluster SPT-CL J0615-5746. The5-arcsec-long arc is the most highly magnified z>10 galaxy known, straddlingthe lensing critical curve and revealing five star clusters with radii ~ 1pc or less. We measure the full arc to have F200W 24.5 AB mag, consisting oftwo mirror images, each 25.3 AB mag with a magnification \mu ~ 60(delensed 29.7 AB mag, M_{UV} = -17.8). The galaxy has an extremely strongLyman break F115W-F200W >3.2 mag (2\sigma lower limit), is undetected inall bluer filters (< 2\sigma), and has a very blue continuum slope redward ofthe break (\beta = -2.7 \pm 0.1), resulting in a photometric redshiftz_{phot} = 10.2 \pm 0.2 (95% confidence) with no significant likelihood belowz < 9.8. Based on SED fitting to the total photometry, we estimate anintrinsic stellar mass of M_{*} ~ 2.4 - 5.6 \times 10^{7} M_{\odot}, youngmass-weighted age of ~ 21 - 79 Myr, low dust content (A_V < 0.15), and alow metallicity of <~ 1\%~Z_{\odot}. We identify a fainter thirdcounterimage candidate within 2.2 arcsec of the predicted position, lensed toAB mag 28.4 and magnified by \mu ~ 2, suggesting the fold arc may onlyshow ~60% of the galaxy. SPT0615-JD is a unique laboratory to study starclusters observed within a galaxy just 460 Myr after the Big Bang.